1. Field of the Invention
This invention is related to Multiple Satellite Communication Systems. In particular, it is related to traffic control in a satellite network for optimizing battery utilization in each satellite.
2. Discussion of Related Art
Terrestrial cellular communication systems are well known. Multiple Satellite communication systems complement terrestrial cellular communication systems to augment traffic handling capacity and service areas where wire or cellular networks have not reached. Satellite systems came into existence in response to the need for efficient and economical mobile communications. In general, the satellites act as a transponder, or "bent pipe", receiving ground based transmissions from one location and beaming the repeated transmission back down to another location after amplification and frequency shifting, as discussed in U.S. Pat. No. 5,448,623, incorporated herein by reference in its entirety.
The amplification process for traffic handled by each satellite, and associated satellite systems, require electrical power, typically derived from gathering solar energy by solar arrays deployed by the satellite. Some of the energy obtained from solar arrays is stored in on board batteries for use at times when solar energy may be unavailable.
The cost of a satellite can be reduced by reducing the energy storage capacity, or ampere--hour rating of the on board battery. In turn, battery capacity can be reduced by controlling traffic related power consumption in the satellite. For this, various ground-based traffic allocation controls have beam implemented. Traffic allocation is desired because, in general, high satellite power consumption requires increased solar power gathering capacity and electrical energy storage, in turn leading to increased satellite mass and decreased reliability.
Solar power reaching the satellite is typically not constant over all portions of an orbit. Variations in the availability of solar power at the satellite arise from the inherent geometry associated with the path of Low-Earth-Orbit satellites around the earth. Eclipsed by the earth, perhaps as often as every orbit, solar power cannot always reach the satellite. Hence, solar power is sometimes unavailable to supply the electrical power required by a satellite during portions of each orbit. During eclipses, the power required is delivered by on board batteries. Battery power is also required when eclipse effects are further magnified by the variation of solar array efficiency with orbital position. This occurs, for example, where the angle between the spacecraft and the sun is low. Hence, satellite based battery power needs to be closely controlled and anticipated during a satellite's orbit to compensate for lack of solar energy during part of the orbit to achieve the satellite's mission.
In the prior art, ground-based traffic allocation is used to manually control the power consumption in satellites.
Such manual controls, if inaccurately implemented, or tardy, may contribute to discharging the satellite battery beyond desirable limits during periods of heavy communication traffic. In general, manual controls of the prior art comprised off-loading traffic from satellites having a low state of charge (SOC) to other satellites having a larger battery SOC. Manual methods were preferred over the more detailed and timely optimization of traffic of this invention because of its relative simplicity. Conventional systems of the prior art generally monitored a power parameter, such as instantaneous, single satellite traffic density, and allocated traffic accordingly. Other prior art also allocated traffic based on telemetry records reporting the historical state-of-charge of a satellite's battery over certain periods of time. However, manual traffic allocation, based on historical data, could be verified for its degree of optimization only long after its implementation, hence could not accommodate dynamic changes in traffic patterns.
Further in the prior art, the traffic allocation decision mechanism in a gateway generally used only local historic information available at that gateway. The traffic routing decision thus produced was based on local, generally incomplete information, generating suboptimal traffic and related power allocation.
Because of the variables discussed, as will be detailed in the present invention, optimizing the allocation of traffic for a particular satellite, needs to be centralized to consider past and future power consumption needs and orbital geometry. As well, a control function that properly weights initial and final conditions of a plurality of variables such as satellite battery state, desired future conditions of the satellite battery depth of discharge, the expected traffic demand, spacecraft system demand, and eclipses is required for optimum traffic routing. Prior art for solving similar these types of dynamic control problems are the Riccati matrix solutions, Convex Programming solutions and Dynamic Programming.
The Riccati matrix solution is a standard linear optimization method for use with dynamic systems. The Riccati solution considers both the initial and final conditions of the system, and then iterates between these conditions until some overall objective is optimized. However, the Riccati technique is applicable only to a linear problem structure. The factors influencing satellite cellular traffic allocation are significantly non-linear in their operation, hence the Riccati technique is limited in its application.
Another approach, the "Barrier" method, is used in Convex Programming, and reduces the computational burden of solving optimization problems using convergence and rate of convergence. These are well established in the context of Convex Programming. In satellite traffic allocation problems, however, convex programming is not very effective generally due to a lack of convexity of the associated data.
Another alternative presented in the prior art, Dynamic Programming, can be used for optimizing dynamic systems with little linear structure. However, dynamic programming is computationally intensive, hence not available in real time especially for large multiple satellite systems. A large number of satellites, with many beams and many separate channels for each satellite controlled by many gateways is relatively complex, perhaps including over one million values, and thus precludes, in many cases, a real time solution with current computing engines.
Another desired result of traffic control for power consumption optimization is limiting the Depth of Discharge (DOD) of satellite batteries, especially when traffic is heavy. Battery life is greatly influenced by DOD. Generally, if battery DOD drops below 60 percent, the life of the battery may be reduced substantially. Maintaining a long battery life is important to sustaining satellite system efficiency.
In light of the above limitations of the prior art, it is an objective of the present invention to provide a central satellite power allocation based on information gathered from a plurality of sources and locations.
It is another object of this invention to provide timely satellite power demand computations to assure the allocation of traffic in response to up-to-the-minute traffic estimates.
It is yet another object of the invention to consider a desired future state of battery charge in a plurality of satellites for optimally allocating traffic to a plurality of communication satellites.
It is a further object of the invention to accept various desired Depth of Discharge (DOD) levels for each of a plurality of satellites for optimization of traffic allocation with respect to these desired levels.
A further object of the invention is to determine an optimum power allocation considering world-wide forecasts of future traffic and measurements (i.e., telemetry) on the battery state-of-charge (SOC) for multiple satellites.
Yet another object of the invention is to partition the solution method of traffic allocation for compatibility with multiple processors, thus facilitating timely parallel computation of the traffic allocation for satellites in a satellite network.
Yet another object of the invention is to optimize traffic allocation among multiple satellites while considering typically higher uplink and downlink path losses during times when a satellite is at low elevation with respect to a gateway and/or terrestrial users typical transmitting/receiving grid point. During these times, when a satellite is below about 10 degrees of elevation with respect to a grid point, the beams have to traverse longer distances in the atmosphere and transmit losses are higher than at other elevation angles.